This invention relates to threshold detection circuitry and more particularly to such circuitry for use in shutting down the electrical power drawn by battery-operated equipment.
There is a need for circuitry which will sense and provide a control signal in response to a monitored signal magnitude falling below a predetermined threshold. More specifically, modern portable electronic equipment such as transceivers often are powered by rechargeable nickel-cadmium batteries or alternatively by lithium batteries which have long shelf lives. In many applications it is desirable that either of these type batteries be employable depending upon the particular circumstances. Unfortunately, lithium batteries tend to become explosive if operated beyond their capacity. The output voltage of both nickel-cadmium and lithium batteries tends to fall through a predictable threshold as these batteries become discharged. Hence, circuitry is required to sense when the lithium battery voltage falls below a threshold and to virtually disconnect the electrical loas from the battery. Furthermore, it is desirable that the threshold voltage at which shutdown occurs be readily adjustable and that the circuitry have a reset feature for enabling the immediate application of power after the battery has been changed and operation at reduced power levels after reset, for instance.
Prior art circuits for performing the foregoing or similar functions sometimes include mechanical relays which are too large, heavy and/or expensive to be utilized in compact, lightweight and inexpensive portable applications. Other prior art solutions require the use of expensive electrolytic capacitors.
Still other prior art solutions while providing power shutdown tend to continue to draw a leakage current after shutdown having an undesirably high magnitude which could have disastrous results in the case of lithium batteries. Moreover, some prior art solutions require an undesirable amount of operating current for portable applications.
While battery-powered equipment is being operated, the shutdown of a particular function such as ceasing transmission by a transceiver will result in an increase in the output voltage magnitude of a dangerously low battery. It is desirable that shutdown circuitry for use with lithium batteries include hysteresis which will continue shutdown of the electronic circuitry even though the magnitude of the battery voltage increases during the shutdown procedure. Some prior art shutdown circuits do not provide this hysteresis.